Tips to build faster in Minecraft creative

WorldEdit - a mod providing copy-paste features, tiled fill, and a bunch more. It can really speed up massive builds with a lot of similar parts.

the /fill and /clone commands. Much more cumbersome to use but available in vanilla.

Debug stick - allows to change states of blocks, e.g. rotate them, set in states that aren't normally available, say, stand-alone corner stair block.

Tweakeroo, another mod with support for fancy placement modes - e.g. easily placing blocks with special facing rules (pistons, logs etc) in desired orientation without need to move around to place them from the correct side; also adjacent or otherwise aligned - placing block against an edge (diagonally adjacent) of other block, or with one block of gap.

Litematica - allows "pasting" saved structures into the world, similarly to WorldEdit but while it's more cumbersome, it's also more "visual" - you get a preview of the operation. Also allows easy migration of big structures between worlds.

"Pick block" - vanilla functionality: middle-click while looking at a block (from roughly breaking distance) to put a copy of it in your hand, so you don't need to switch between blocks manually, you can "pick" from the build.

Saved Hotbars. Another vanilla function. Set up your hotbar with a "family" of blocks you use together and press C + [number] (1 to 9). Now if you press X + [the same number], no matter what you had in your hotbar, it's replaced by the same set. Preparing a couple hotbars with "palettes" for particular purposes (also saving the debug stick so you don't need to /give it to yourself whenever you need it), really speeds up the work.