If so, how significant is it? The main domain is on AWS/IIS with the impression that the subfolder contains a WordPress site hosted at SiteGround.
I host 100s of high traffic, high speed, WordPress sites.
If a client asked me this question... I'd say...
This type of spaghetti setup is okay for a hobby site. Not for a real + money generating + pay your bills type of project.
For real (money generating) projects. Keep tech simple.
I run straight up Apache + FPM PHP + sites I host load test at roughly 1.1M request/minute throughput. You read that right.
There's all sorts of myths about what's required for fast sites.
My guess, 99.999% of everything written about fast sites is written by people who've never run a high traffic site or don't understand how to setup sites to isolation + fix slow points.
Do yourself a favor + hire someone smart to host your site.
In the long run, you'll save a truckload of money.
Reverse proxies can have a negative impact on page speed. It is difficult to completely answer your question without additional information such as:
Why is there a reverse proxy at all?
How significant is the performance degradation?
If you are interested, I can get on a call with you.