Will Windows 10 on Bootcamp slow down my MacBook Pro?

Your 128GB is too small to install OSX and Windows and still have room for data and apps/programs.

To answer your question directly. using Boot Camp to install Windows creates a completely separate bootable partition for Windows. When running Windows there is no impact on OSX because OSX is not running concurrently with Windows. When running OSX there is no impact on Windows because Windows is not running when OSX is running. So there is no impact from either OS when either is running.

If you leave very little free space it will slow your computer. But exactly how much free space you should leave on each partition is not exact. Each partition needs enough space for the OS, apps, data, swap/page files, system operation... A number that is often used is that OSX requires 15% free space. Windows likely needs a similar amount.

I'll say it again. 128GB is not enough for both OSX and Windows. My rule of thumb is to allocate at least 100GB for Windows alone. This does not leave enough space for OSX to run if you want to do anything.

I have a MBP mid-2012 13-inch with a 500gb SSD. and 16gb or ram, Intel Dual Core i5 CPU

I created two partitions with bootcamp of 250gb each. One for OS-X and One for Windows-10.

250gb. partitions each is what I would recommend for each operating system. This will leave you room for apps and files.

It will not slow down your MBP. With bootcamp you choose which OS to boot into on start up. One does not effect the other.

I ran the same benchmark test in both operating systems and Windows-10 scored higher benchmarks than OS-X El Capitan believe it or not. How ever just by a little. ( CPU, Memory, Graphics, Nova bench, Geekbench, SSD Benchmarks etc. ).

OS-X and Windows-10 only use about 15gb. each of disk space so it is possible to create two 60gb. partitions on your MBP and load OS-X El Capitan and MS-Windows-10 however you will not have much room left for apps and files.

I recommend you upgrade your SSD to a 256gb. or 500gb. best for boot camp.

120gb partitions each for each operating system should be the minimal with 250gb. partitions being recommended.

MS-Windows-10 performs very very well on a MBP. Everything works just like a Windows PC laptop.

You will need to install the latest bootcamp drivers for your MBP however this step is very easy to do.

The instructions are all in the boot camp app when you do the install.

Use Windows 10 on your Mac with Boot Camp - Apple Support

Windows does not support running from an external drive unless you get the Enterprise version of Windows.

I add my support of Bob's conclusion. Entry-level Flash drives are simply not large enough for BootCamping. I have Win 7/64 on my iMac with a 1TB rotational drive and the Win7 BootCamp partition is 160GB, one-eighth of my total capacity. It runs fine but I would not want it any smaller.

The other complication is that BootCamp will not work from an external drive.

You can buy an SDD upgrade from OWC

it is $400 however. Not cheap.

https://eshop.macsales.com/shop/ssd/owc/macbook-pro-retina-display/2013-2014-201 5

May not be worth it unless you use your MBP for work, school or home office in which case you can write off your expenses in your yearly income tax form.