user135962: I am working on a parser for the blk.dat files being returned by the Bitcoin Core node. It seems to slow down considerably after a point and I am pretty sure it's because the number of transactions is increasing in the files. My question is, how many transactions do these files contain and is there any way to figure out how many there are without parsing?

user48884: No. blk*.dat files are limited by actual size on disk, not number of transactions. Blocks are added to them until they reach a maximum size, not whether they exceed a block or transaction count. As transaction sizes are variable, it is not possible to know how many transactions are in a blk*.dat file without having parsed it first.
However, as Bitcoin Core has already done that parsing for you, you can use the LevelDB database that it creates which contains that information. This answer provides an explanation of the format of the LevelDB records in the block index (the database stored in blocks/index).

135962: <p>I am working on a parser for the <code>blk.dat</code> files being returned by the Bitcoin Core node. It seems to slow down considerably after a point and I am pretty sure it's because the number of transactions is increasing in the files. My question is, how many transactions do these files contain and is there any way to figure out how many there are without parsing?</p>
 48884: <p>No. <code>blk*.dat</code> files are limited by actual size on disk, not number of transactions. Blocks are added to them until they reach a maximum size, not whether they exceed a block or transaction count. As transaction sizes are variable, it is not possible to know how many transactions are in a <code>blk*.dat</code> file without having parsed it first.</p>
<p>However, as Bitcoin Core has already done that parsing for you, you can use the LevelDB database that it creates which contains that information. This <a href="https://bitcoin.stackexchange.com/a/67569/48884">answer</a> provides an explanation of the format of the LevelDB records in the block index (the database stored in <code>blocks/index</code>).</p>
