Good tip from @twin_savage: even an LTO tape & drive working perfectly will have brief pauses every few minutes as the tape switches directions for another pass. How often are you seeing (or hearing) it?
LTO-4 has a maximum speed of 120Mbps. If you’re seeing around 234Mbps most of the time, your “bigfile” is something that compresses quite well like a text file, disk image, or SQL dump. If it can be compressed more than 2:1, you’ll hit compression speed limits on your LTO drive throughput, and the tape will spin down (possibly quite a lot) because of it. Much better to compress your bigfile (or any other data) in software with pigz/xz/zstd/etc. first, before sending it to tape to avoid this.
Is the buffer staying in the double-digit “% full” range the whole time?
As for other a few suggestions, see my prior post: