As an 8/32 device, the tl-wr1043nd v1 is below minimum system requirements for OpenWrt[0] - and yes, its 400 MHz mips 74Kc SOC is slow, very slow (contemporary ath79 SOCs run at up to 750 MHz - and still run out of steam around 175-200 MBit/s). Keep in mind, while this device was designed and sold on the market, 25-50 MBit/s WAN connections were the king of the world. Yes, you might push[1] it during speedtests, but as walterav said, pushing it beyond its hardware limits results in very jagged throughput results and very bad latency and jitter.
Coping with these line speeds requires considerably better/ faster hardware than this 12 year old early draft-n specimen[2]. While you don't need to go quite as far, So you have 500Mbps-1Gbps fiber and need a router READ THIS FIRST does explain the underlying issues and how one could cope with them (e.g. the rt3200 would be a sensible upgrade for reasonable money, leaving the door open for SQM and some moderate future speed increases).
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[0] read, it's explicitly unsupported by OpenWrt 21.02.x and hasn't run well in 18.06.x or 19.07.x either, its prime has passed almost half a decade ago. Modern 802.11ac devices do provide significant performance improvements at all fronts, WAN speed, luci interactivity, VPN throughput, wireless performance and range, not to forget 5 GHz support and 'comfortable' amounts of flash and RAM.
[1] and even more by enabling software flow-offloading, but that comes at its own share of penalties (no sqm, no traffic accounting, often weird quirks and issues).
[2] yes, I do still own one myself