site slow

The time taken to parse that extra directory is trivial, insignificant, too small to measure.
I think that using a subdomain is often better than using a directory. So in your example I would suggest https://abc.123.com/ rather than https://123.com/abc/
There are various plugins for cache managemnt, you might look into these, you must measure what they do for you rather than what they promise.
The quality of your hosting is also very important.