Q: MacBook Pro Running Slow

I have a Mid 2012 MacBook Pro, just recently upgraded to Sierra (10.12) and I've been having performance issues for quite some time. Initially I thought it was just that I had too much stuff on my computer because I eventually started getting Startup Disk almost full messages, so I cleared off as much as I was able while on vacation, though still only have about 25 out of 500 GB available and not sure when I will find the time to sift through more. This seemed to help a little bit, but overall the system has still been running slow – startup takes forever, apps taking a long time to open, websites take a long time to load, mail takes forever to load or access a contact, and even typing a paragraph like I am now can be slow waiting for the computer to catch up to my input (even in offline apps). The problems started before I upgraded to Sierra, however, the upgrade has not helped, may have even slowed things down more. In trying to find solutions on the forum here, I have tried a few things with minimal results. I tried starting in Safe Mode, which did seem to function better, but of course, when I logged back into normal mode, things were back to, well, normal. Discovered my Activity Monitor and saw that Symantec (SymDaemon) was at over 100% CPU, so forced that to reset and now it seems to be staying much lower. I have heard that Norton can slow down Macs, but my Dad (who is in the computer security industry) tells me I should keep it on there, that even Macs (his computer of choice) are at risk these days. Norton did once catch something on my system a few months back, so I am hesitant to remove it completely. Since resetting SymDaemon, I have been checking in on the Activity Monitor and have found other processes occationally up there on the CPU usage and have googled them and tried to reset them. One was logd, which would not reset and suggestion I found was to just restart computer, which seems to have knocked it back down for the time being. Photolibraryd was another one, which I found may be happening as my photos get recataloged under Sierra? Though it has bumped itself back down. Right now as I watch it, nothing major seems to be eating up CPU resources, occationally something will jump up to 30-80 and then back down, yet, still having performance issues. I also removed all startup items from my account by hitting the minus to completely remove them from the list. Still no change. Part of me feels like I need to just wipe my computer and start from scratch, but I have way too many playlists (many of which I use for work), photo albums and other things set up that would take forever to rebuild, which I simply do not have time for. And in the past when I've tried wiping everything and then migrating things back over from Time Machine, it seems like the problems follow me. I have been in the habit of running the Disk Utility FirstAid 1-2 times a week for quite some time, but that does not seem to do much either. I am at the end of my rope, something must be done because I am wasting too much time sitting and waiting for my computer these days. I welcome any further suggestions! Thanks!