what is this white bar at startup?

Called a progress bar and appears under the Apple during startup.

There are many reasons why a computer may be slow. One of the primary ones is insufficient RAM installed. The computer's performance will improve with more RAM such as 8 or 16 GBs depending on what your model supports. Please read Writing an effective Apple Support Communities question.

Possible Fixes for High Sierra

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

Be sure to backup your files before proceeding if possible.

1. Shutdown the computer, wait 30 seconds, restart the computer.
2. Resetting your Mac’s PRAM and NVRAM
3. Reset the System Management Controller (SMC)
4. Start the computer in Safe Mode, then restart normally. This is slower than a standard startup.
5. Repair the disk by booting from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volume entry from the side list. Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and return to the Utility Menu. Restart the computer from the Apple Menu.
6. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button under Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.
7. Download and Install 10.13.1 High Sierra Update
8. Reinstall OS X by booting from the Recovery HD using the Command and R keys. When the Utility Menu appears select Reinstall OS X then click on the Continue button.
9. Erase and Install OS X Restart the computer. Immediately after the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears:

1. Select Disk Utility from the Utility Menu and click on Continue button.
2. When Disk Utility loads select the drive (out-dented entry) from the Device list.
3. Click on the Erase icon in Disk Utility's toolbar. A panel will drop down.
4. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply button, then wait for the Done button to activate and click on it.
6. Quit Disk Utility and return to the Utility Menu.
7. Select Install OS X and click on the Continue button.

Ways to help make a slow Mac faster

1. 17 Reasons Why Your Mac Runs Slower Than it Should
2. Slow Mac Performance? This Article Solves It!
3. Fix slow start-ups in OS X | MacFixIt - CNET Reviews
4. How to fix slow shutdown and startup times. | MacTip.net
5. 6 Easy Tips to Speed Up OS X Yosemite on Your Mac
6. OS X El Capitan- If your Mac runs slowly
7. Visit The XLab FAQs and read the FAQ on Performance
8. Tips to Fix Issues that affect Mac Performance
9. Avoid using any third-party cleanup software. Usually, this software does more bad than good. You don't need it. All computers become slower over time even under normal use. Experienced users erase the hard drive and do a clean install from scratch from time to time; or whenever installing a major OS upgrade. Doing so means you must maintain regular and multiple backups.
10. If you have enabled iCloud Disk and are storing your Documents and Data in iCloud, then consider turning that off and signing out of iCloud. It can slow down the computer considerably. Please see the following from the user, fotomac: "The solution was to SIGN OUT of iCloud and my problem STOPPED! NO MORE SPINNING BEACHBALL! My computer's speed increased to what it should be and all my Apps now work!"
11. Add more RAM or cut back on the number of concurrently running applications and utilities. Remove unnecessary anti-malware software and any software that promises to clean your Mac. Check for runaway processes: Runaway applications can shorten battery runtime, affect performance, and increase heat and fan activity. Also, visit The XLab FAQs and read the FAQ on dealing with The Spinning Beach Ball of Death.
12. The Ultimate Fix: Backup everything, erase the drive, reinstall OS X, and restore your data from the backup. Reinstall third-party software from original media/scratch.

Looks normal to me.

Try running this program in your normal user account, then copy and paste the output in a reply. The program was created by etresoft, a frequent contributor. Please use copy and paste as screen shots can be hard to read. On the screen with Options, please open Options and check the bottom 2 boxes before running. Click “Share Report” button in the toolbar, select “Copy Report” and then paste into a reply. This will show what is running on your computer. No personal information is shown.

Etrecheck – System Information