I've been in networking now for 38 years and I have that 'slow network performance' is 99.9% related to packet loss/drops. Some where in your path packets are being dropped, maybe via congestion or something similar. Or bad HW, etc.
1st step is probably to look at the stats of the switches to see if they show any errors, dropped packets, etc. You could also look at the traffic via packet capture. Look at it via port mirror to an analyzer. Then you need to read the capture(s) along the path.
Maybe easier said then done. Hopefully stats show you something.