Computer very slow lately

"Slow" is subjective. Can you post numbers or a link to some performance monitoring data? Something like monit or munin will generate graphs that we can all understand. I don't know how to get conky to capture data that can easily be shared.

Systems performance comes down to shared use of:
* cpu
* ram
* network I/O
* disk I/O
If those things are below 80% utilized, then look towards misconfiguration or hardware faults. For those - I'd ask, what do the system log files say? A common reason for slowness is a failing HDD or cable or controller or bad switch port or failing router or .... failing RAM or ... you get the idea. Sometimes the answer is simple - reseat the RAM, check all SATA coectors, run some RAM tests - sometimes 1 stick will fail, but in a 16G box, you'd never notice without checking the log files.
Sometimes a bad USB device can destroy overall performance too. Look for USB fault messages - or overheating or .... there are hundreds of possible answers.

So ... what do that log files say? https://help.ubuntu.com/community/LinuxLogFiles