No pacing problems, Xen unproblematic? I can't say that anywhere near approached my experience with the game. Whatever pacing problems HL2 had, HL1 had them just as bad for me (if not worse in spots). And Xen, christ. It was the least polished section of the game by far, just large expanses of janky jumping puzzles (though a few I liked), bland corridor and "staircase" crawls, as well as boss fights that ground on my nerves quite a bit. HL2 bests HL1 in most respects for me, though the latter certainly has chapters I am very fond of (Blast Pit [my favourite, in large part thanks to the branching objectives structure, it's inspiring my own level design at the moment], On a Rail, Apprehension, Questionable Ethics, Surface Tension, and "Forget About Freeman!". Half-Life was all about that second act).