This is a huge question. The simple replacement of cabling would not normally cause this.
And the definition of poor lan performance is as wide-open a question as "my car is making funny noises, what should I do"
I recommend that you collect performance complaints and see them occur yourself, be able to reproduce the complaint, if it's a single app/network, you've narrowed it down, if not, is it app specific, a single server or server farm, building, room? Lots of stuff to divide and isolate what the real problem is.
1st - Turn on Logging, log everything you can, short of debug all outputs (might slam the gear CPU to the point of failure, non-repsonse)
Classically I'd recommend the Cisco-blessed bottom-up approach.
Layer 1. Check all port/interfaces for physical layer errors, misconfigurations, speed/duplex mismatches, etc. Fix, recheck, etc. Are you seeing hi-cpu loads on any gear in your network.
Layer 2. Look at ARP tables,default routes. Up/down situations on interfaces. Spanning-tree, check status, logs, debugs, any loops, execessive BPDUs? Flapping interfaces etc.
Layer 3. Systematicaly ping from an end-station experiencing the problem through your network, any huge latency issues? See where it slows and check that device. Are you getting a ton of broadcasts - sniff segments and see. Apply remedy as you find issues. Run traceroutes, look at routing tables in each router... anythink look funny? Fix and move on.
Layer 4 - 7 -- Sniff, baseline tftp/ftp tests from a work station to an access-lever switch and then out 2 or 3 devices in your network. Any major changes in delta times? Look for heavy ACL activity at each device.
This is just a very very basic summary of what you need to do. Use CCO and do searches on each level of troubleshooting.
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