Speaking of a certain solo you want to speed up;
Do not use metronome until it %100 gets into your muscle memory and you can play perfectly and expressively in slow tempo(or No-Tempo) Until then just play slow and
"FOCUS" on your right and left hands watch if there is any uneconomic excessive movements, the angle you hold the pick, how much of the pick is flapping outside..etc. The more you focus on anything the more experience will be permanent.
While focusing, spot and constantly eliminate any muscle tension in your arms and hands(this is HUGE) Your goal is to have an incredible lightness in your fingers. If you feel exhaustment or pain in your fingers after a small solo, chances are you are playing against a lot of muscle tension, which can lead to injury and limit your speed. (don't confuse this with the mellow tiredness of a muscle group after a workout)
"Listen" the sound you create, make it sound flowing and "BEAUTIFUL" in any speed, as if even with that speed it would play on radio and people would love it. Every note should have a definition.
When it starts to sound "beautiful" and correct, then preferably with a metronome little by little increase the speed without compromising that beauty and quality. But speed should come to you as a result of excellent conditions, and not you are trying so hard for it. All of a sudden you will feel like you can play a little faster with any extra efforts and tensions. Then you never have fear of playing the solo right or wrong for that speed, it will always be right. it will not be one big moment in the song that you feel the fear before solo if your gonna pull it correctly now or not.
Then you will get stuck on certain parts of solo, analyze them whats going on(which picking mechanics you use(up/down), are you changing strings with an up or down stroke ..etc) and isolate and only work on that 2-3 note spots maybe 1000 times(this is called efficient practising) then slowly incorporate everything together by the time.
After a while you will be saying to yourself "It's so easy, how come I couldn't play like this on the first time"