I am trying to get the sequence of 200bp upstream of all zebrafish transcripts.

I am using the following code:

library(GenomicFeatures)
library(BSgenome)
library(BSgenome.Drerio.UCSC.danRer10)
library(biomaRt)
txdb <- makeTxDbFromUCSC(genome="danRer10", tablename="refGene")
transcripts <- transcripts(txdb, columns = c("tx_id", "tx_name"))

# Get TSS
tss <- resize(transcripts, width = 1, fix = 'start')
# Get 200bp upstream of the TSS. 
upstream <- flank(tss, 200)
# Trim any range that is out of chromosome boundaries 
upstream <- trim(upstream)

upseq <- Views(Drerio, upstream)

This seems to work perfectly and I get:

> upseq
BSgenomeViews object with 16586 views and 2 metadata columns:
                  seqnames           ranges strand                       dna |     tx_id      tx_name
                     <Rle>        <IRanges>  <Rle>            <DNAStringSet> | <integer>  <character>
      [1]             chr1 [ 11767,  11966]      + [AGACGGGGAA...CGGTGACACA] |         1    NM_198371
      [2]             chr1 [ 27488,  27687]      + [TGATGATGAT...TCGTGCAGTC] |         2 NM_001017766
      [3]             chr1 [ 36534,  36733]      + [CTCTGTCTCT...AGTGTCTGCT] |         3 NM_001017577
      [4]             chr1 [109598, 109797]      + [AGTAAGCGGG...GAGTTTGTGG] |         4 NM_001089558
      [5]             chr1 [119760, 119959]      + [GATATATGCA...TTCAAACAGA] |         5    NM_131819
      ...              ...              ...    ...                       ... .       ...          ...
  [16582] chrUn_KN150696v1   [ 7666,  7865]      + [GAGCCTTGAG...TTTGTGTTTT] |     16582    NM_131603
  [16583] chrUn_KN150699v1   [ 8272,  8471]      + [GTTCGGACAA...CTTGACTCGA] |     16583    NM_207058
  [16584] chrUn_KN150699v1   [15187, 15386]      + [AGCTGAGCTC...CTGATGTAGA] |     16584 NM_001320205
  [16585] chrUn_KN150706v1   [ 4311,  4510]      - [ATACTCCACT...NNNNNNNNNN] |     16585 NM_001199970
  [16586] chrUn_KN150708v1   [31564, 31763]      - [CGTTACTCTA...AGACACCACA] |     16586 NM_001256832
  -------
  seqinfo: 1061 sequences (1 circular) from danRer10 genome

Now, I'd like to get those sequences in a character vector. I thought I could do:

sequences <- lapply(upseq, as.character)

This works but takes several minutes, which makes me think I am doing something wrong. Could you suggest a specific way of extracting those sequences?

You could do DNAStringSet(upseq) to stay in the Bioconductor universe, or as.character(upseq) to work with plain character vectors.