Dave here are some numbers.
OPs please don't bash me for using MaxSea I was using it before O got to where it is now and have a large investment in Jeppersen C-Map charts
that are superb. O is now getting very useful and I am finding it more reliable. I have an O-charts license
and tried a few charts and will go down that route
when my Jeppersen charts become outdated or I move out of the range of coverage that I have.
Timings with approx 75 track sections imported from MaxSea of between 500 miles 0.1 miles in length
Reduce track (step 10) of 46.95 miles - 10 minutes
Merge two, reduced, tracks of 30 & 46 miles - 10 seconds
Reduce track (step 10) of 60 miles - 14 minutes
Merge two, reduced, tracks, 60 & 76 miles - 10 seconds
Merge two, un-reduced, tracks 46 & 38 miles - 1 hour (more or less)
merge three reduced tracks 137, 37, .3 miles - 10 mins +
The total size of the GPX file is about 80M as exported from MaxSea, it imports quite quickly to O, but processing it to reduce the track points and merge the daily tracks is, as you can see, very time consuming. Performace improves as the tracks are compressed and merged so that there are less track sections.