When I restart a job mid-file, lets say “Line 600,” the router is moving at a snails pace. Crawling until it goes like for 3 minutes. Maybe to a certain line. And then it resumes at full speed. Happens on many files of mine. Am I the only one? No issues in the past.
Gsender 1.4.12
More than likely your gcode does not request a speed change right after you restart the job. When it finally gets another speed change request it returns to normal operation.
You can (don’t know if it is practical) insert a speed command into the gcode file or, and I don’t know if that works, insert a speed command on the console before you restart.
@Clt49er I’m not at all sure that I have a solution, but I do have a question. What post processor are you using to create the gcode?
@Clt49er If you are using UGS GRBL inch switch it over to UGS GRBL mm
I went through this issue back in March, and when I switched the post-processor over to
UGS GRBL mm, the problem was solved. Link to my thread below
Start from Line Moving very slow - gSender / 
Bug - Sienci Community Forum
*I think the last time UGS GRBL inch worked fine was on v1.4.10 or before. I can’t remember
*edited my last sentence for clarity
@Swinly Tks, Lonnie. I didn’t put forward a solution becuase I was interested in @Clt49er answer. I think that that the problem can occur even if grbl inch is used. I’m not sure, though. It seems to me that, since grbl is natively metric, when the “start from line” feature is used, it goes to its default. So, if the machine was moving in inches, it is now moving in MM - much smaller increments.
Yep. I have been using grbl inch.
@Clt49er Sienci may be able to confirm my suspicions, but I believe that’s the problem.
Using grbl inch, there is a G20 command at the start of the gcode file that tells gSender that all the movements that follow are in inches. So, for the sake of argument, a G1 X5 moves the router 5" to the right.
I am thinking that, in the event that you use “start from line”, there is no G20 command, so gSender defaults to metric. Now, the G1X5 command only moves the router 5mm to the right.
I just tested this theory on an old gcode file that I had that used grbl inch as the post. I stopped it mid-job, then used start from line to continue. It ran very slowly. It was now moving 80mm/min when it had been running 80ipm/2000 mm/min.
I tried to input a G20 command in the console before using the start from line, but it didn’t solve the problem.
Please don’t take this as gospel. It needs confirmation or refuting from @KGN.
Did you try G20 followed by the speed that you were expecting? G20 enter G94 xxxx enter
As a side note, I find it odd that, unless the controller was reset, that the controller would switch to mm. I can certainly see it powering up in mm as default …
@Jens Keep in mind that I never said that I was right. 
I did not try entering anything other than G20. This never happens to me as I use grbl mm as my post processor.
LOL … and I never said you were wrong 
I was just grasping at straws and I have no idea if it would make any difference whatsoever. I still am baffled as to why the controller would switch to mm unless a reset was involved (I am not expecting an answer, just thinking out loud)
@Jens “I was just grasping at straws and I have no idea if it would make any difference whatsoever.”
Me too.
This is an issue in 1.4.11 or 12. Forget when it changed. I reported it between Christmas and New Years. Reverted back to 1.4.10 and it works fine. Definitely an issue in the update.