Full SSD equals slow Mac?

If you are booted up to your El Capitan partition then your mac only has 10GBs of free space, regardless of what free space is on other partitions, the mac does all its housekeeping and reading and writing of system files on the disk/ partition it is booted to.

The mac does not use other partitions from the boot drive. If you want to save files to other partitions then you must tell it to do so.

The next thing is that you need to have more free space on your disk/ partition, the general consensus of opinion is that you should never let your macs boot disk run below 15-20GBs of free space. It will begin to run slow, and if it gets very low on free space may just lock up. That could prove very problematic, the mac needs room to breathe, it is constantly writing and moving files around. So any files you do not need on a daily basis you should copy over to an external disk or those partitions on your SSD. Movies, images, music usually take up the most space, so start by copying them over. When you are sure they have copied over successfully, then you can delete them from your El Capitan disk/ partition.

The other issue is some SSD controllers will slow performance as the SSD approaches 50% used storage. Read the detail Samsung 850 EVO small print to see if they do this with their SSD drives.