Garmin Forerunner Current Pace

In your set up of the screens you can pick which pace you want - it sounds like yours is on 'average pace' which calculates an average pace for that lap.  I haven't looked into it for a while but there is usually a little delay in reading out pace (I have the 310, used to have the 305) and found it could move quite quickly - not so slow as you mentioned.

But i suggest you take a poke around the settings and screen display options and you'll see the various pace options when you scroll through.

I'm not exactly sure how it calculates the pace but my guess is the average distace covered over the last 20-30 seconds.

I do know that the 210 version of this allows you to display current, average, and lap pace depending on what you select.  Maybe it is as easy as changing the type of pace you are displaying.

Look up the manual for your 205 at:

https://support.garmin.com/support/

The instantaneous pace is rather tenuous.  If Garmin gave you the actual instantaneous pace it was receiving the value would be jumping all over the place- so it's about a 12 second average  (read that somewhere)... and it still jumps around quite a bit.  I did a test with two 405's on my wrist, both set up the same, and often the instantaneos pace was 1 min/mi different between the two.  Sometimes it was very close- within seconds.

Lap pace may be a more useful metric- and even that is probably only accurate to 10sec/mi or so.

I currently have a 210 but used the 205 for several years.  I've used several other Garmin models and they all jump around a bit.  It can be a bit frustrating but over time you learn how to interpret the pendulum.  It not perfect but gives you some sense of your pace.

For comparison, when I run with my daughter (305) or my wife (310xt) I'm constantly asking what pace they have.  Sometimes they are pretty close and sometimes they are swinging out to the edges of the range.   

I stopped using this function, it's very inaccurate. You can be running a 5 minute mile one second, and a 12 minute mile a few seconds later.It also seems a lot more sensitive to bridges, trees, etc. anything that could briefly interrupt satellite signal.

I prefer to do auto-lap and show lap times. You could try playing with the pace smoothing settings if you're doing zone training.